Mozilla Firefox 21 is a web browser developed by Mozilla Foundation , available for download in full install version for Windows platforms from legacy sources. This version of Firefox was officially released on 2013-05-14 for public access and a brief summary of important updates and change log pertaining to this edition can be read below.

Firefox 21 released with Enhanced Three State User Interface for DNT, multiple provide supporting Social API, Suggestion to speedup the Firefox startup time and implementation of Firefox Health Report. 

Changes in this edition includes the ability to get back removed thumbnails in a new tab, optimization of graphics performance, and removal of E4X support from Spidermonkey.

In the bug fixes part clearing browsing & download history has been segregated to avoid accidental removal of browsing history and errors related to function keys non-performance.

There are still some known issues remain unfixed like unreadable italic text, full screen playback support during broken youtube link, crashing of browser while trying to start with locked profile and duplicated downloading while private browsing is activated.